Analyzing ozone formation conditions for aged ship plumes in a local scale chemistry transport modeling study

Freshly emitted ship exhaust gas contributes to the local reduction of boundary layer ozone levels through the reaction of the emitted nitric oxide with ozone and the formation of NO₂. However, after a certain time of plume aging and depending on meteorological conditions as well as the availability of other oxidizing species, new ozone will be formed from NO₂. This implies that ship emissions will act as an ozone source in some distance from the ship. Considering the new and stricter recommendations for ozone limit values from the World Health Organization, it becomes important to analyze and quantify conditions for ozone formation through ship emissions in coastal areas. This study investigates impacts of various parameters like the season, diurnal cycle, cloud coverage and emitted substances on the rate of ozone destruction and formation in ship exhaust gas plumes with the local scale chemistry transport model EPISODE-CityChem. First results show the highest potential of ozone formation under clear-sky daytime conditions during the summer season. A total solar radiation >300 Wm^-2 is necessary for ozone formation in Central European latitudes. A maximum ozone formation rate was found for a plume aging time of ~4 hours. A lower NO/NO₂ emission ratio as well as lower CO emissions show tendencies of stronger O₃ formation in the aged plume. The highest ozone formation goes together with a maximum loss rate of carbon monoxide (CO) and the hydroperoxyl radical (HO₂) as well as the maximum formation of the hydroxyl radical (OH). This indicates that the effects of ship emissions on coastal air quality is highly variable and depends, beneath meteorological influences, on the distance of the ship to the coast and the mixture of pollutants in the plume.